Depends on how much work you are willing to do. The small algae-eating shrimp don't put a heavy bio load on the tank. And some types of shrimp like to be crowded. We've got 5 Amanos (3 berried females) in a 1 gal. tank. We've got about 15 ghosts in a 2 gal. There are a LOT of Red Cherries in a 2 gal. (cause they keep breeding). The 10 gal. has maybe 50 shrimp. We do a minimum of 10% water changes every week and feed them all about every other day (spirulina powder, boiled collard greens, shrimp powder, Golden Pearls, veggie flakes, defrosted glass worms, defrosted blood worms, or algae when we can find smoe) and they are all breeding and doing just fine. However, all the tanks are heavily planted, are species only tanks, have sponge filters, oak leaves, sticks, thick layer of mulm, logs, java moss. The tiny Opae-ula have been found with 100/gal (that's the estimate anyway).